An antireflection coating typically consists of a multilayer stack comprising interferential thin layers, generally of alternating layers having high and low refractive indices. When deposited on a transparent substrate, the antireflection coating has the function of reducing its light reflection, and therefore increasing its light transmission. Therefore, a coated substrate has its ratio of transmitted light/reflected light increased, thereby improving the visibility of objects placed behind it.
An antireflection coating can therefore be employed in many applications, for example, for protecting a painting lit by a light placed behind the observer, or for constituting or forming part of a shop window, so that articles displayed in the window may be more clearly distinguished, even when the internal lighting is low compared with the external lighting.
The optical performance of an antireflection coating can be assessed by various criteria. An antireflection coating is considered to be effective if it can lower the light reflection of a substrate made of standard clear glass down to a given value, for example, 2%, or even 1% and less. The colorimetry of the resulting glazing is also important. Most often attempts are made to ensure that the coating does not substantially modify the color appearance of the bare substrate and, in general, to assure that the appearance is as neutral as possible.
Furthermore, secondary criteria may also be considered, depending on the application, in particular, the chemical and/or mechanical durability of the coating or its ability to undergo heat treatments without deterioration. Another important factor is the ability to produce the coating on an industrial scale; which depends on the deposition technique used, on the cost and nature of the constituent materials of the multilayer stack, on the cycle time needed to produce the coating, on the size and shape of the substrate, and the like.
Optimizing, at least from the optical standpoint, the thicknesses and refractive indices of the antireflection coating layers has been the subject of numerous publications. With regard to four-layer antireflection coatings, which offer a good compromise between the desired antireflection effect of the product and its manufacturing cost, mention may be made, for example, of U.S. Pat. No. 3,432,225, describing multilayer stacks of the (ZrO2/MgF2)2 type, U.S. Pat. No. 3,565,509, describing multilayer stacks of the (CeO2/MgF2)2 or (CeO2/SiO2)2 type, and the publication “All-oxide broadband antireflection coating . . . ” by N. Buehler et al., 15 Aug. 1998 (Applied Optics Vol. 27, No. 16) describing (TiO2/SiO2)2 multilayer stacks.
This latter type of multilayer stack is advantageous since it uses, as the constituent material, high-index layers of titanium oxide, which effectively have an index of about 2.45. This material is advantageous in that it can be deposited in a known manner by magnetically-enhanced reactive sputtering, in an oxygen atmosphere, using commercially available low-cost titanium targets. Its use, however, is not devoid of drawbacks. For example, although incorporating it into an antireflection coating allows very low levels of reflection to be achieved, it is not optimal with regard to the “stability” of the appearance of the coated substrate in reflection. By “stability” of the appearance of the coated substrate in reflection is meant the following two points:
1. The stability as a function of the angle of incidence, i.e., it is preferable for changes in reflected intensity and in tint in reflection to be as small as possible when the angle of incidence changes from being normal to the glazing to a more grazing angle of incidence (or, more generally, from a given angle of incidence, corresponding to the most probable angle of incidence at which the glazing may be observed, to an angle of incidence which is different); and
2. The stability as a function of variations in the thicknesses of the layers, at a fixed angle of incidence, i.e.,. That the appearance in reflection remains almost unchanged even though there is, depending on the production tools available, a certain variability in the thicknesses and/or refractive indices of the layers actually deposited.
Thus, there is a need for improved antireflective coatings. Indeed, stability with regard to the angle of incidence is becoming more and more of a requisite for a variety of applications, such as vehicle windscreens or display screens, for example.